Sorry to get back to you late here. I would imagine that importing from phpBB into bbPress will associate the users into bb_users. Being integrated, bbPress uses wp_users. That might cause some problems. Maybe you can edit the phpBB => bbPress importer to put the users into wp_users versus bb_users. That being said, *****BACKUP EVERYTHING FIRST******

You might want to install a test install on your server and try it there first as I am not sure what the consequences of doing it would be. Nothing too painful, but it might make you have to make a few changes to make sure the posts get associated with the right authors! One way to find out! Good luck!